Inverters are utilized in AC motor drive, utility interface, and an uninterruptible power supply (“UPS”) applications as a means for converting DC to AC electrical power. A traditional inverter generates a low frequency output voltage with controllable magnitude and frequency by programming high-frequency voltage pulses. The high frequency voltage pulses open and close switches to expose a load to pulses of DC current. An inverter of this type is said to be using pulse width modulation (“PWM”). Timing, duration, and voltage of the pulses simulate the peaks and troughs of traditional sinusoidal alternating current. Where the load has an inherent inductive nature, such as windings of a motor, the pulses approximate the sinusoid without significant high frequency harmonics.
To handle larger and larger input voltages, larger switching transformers are needed. Where silicon fabrication has not kept up with the need for greater power, a three-level inverter topology has arisen. The topology equally divides two input voltage sources, thereby allowing twice the total voltage at the output for the same capacity transistor. The inverter was further refined for applications that do not have divided input voltage sources to have instead a series connected capacitor bank defining a neutral point-clamped three-level inverter.
The three-level inverter is one of the most popular topologies for three-phase multi-level voltage source inversion. The advantages of the three-level inverter are:
1) Because of the redundancy of the switches, voltage across any one switch is only half of the DC bus voltage;
2) Switching losses are cut in half due to the reduced harmonics present in the output wave forms for the same switching frequency; and
3) The power rating increases.
The literature recognizes certain drawbacks, as well, in the three-level inverter. Such inverters require complex control circuitry, each of the redundant switches add to the price of the inverter, and the charge at the mid-point between the two DC linking capacitors can accumulate when switching is not balanced.
In many applications, including for example, energy storage flywheels coupled to synchronous motors, failure of the inverter will cause the driving motor to impart an unequal torque to the flywheel. Such unequal torque, especially at very high revolution rates, might be catastrophic to the flywheel. However, the inverters will only work as long as the switching components within them will work.
The three-level topography is configured to allow current to pass through two distinct switching paths for each activation state. In every instance there is a “best” solution and a second “better” solution. Because of this inherent redundancy and because of the strength of the switching products the three-level topography of the three-level inverter has inherent redundancies that will allow it to be used, if properly driven, for a fault-proof inverter. However, without a driver that will quickly recognize a fault, in turn, disabling one of two switching paths, diverting current only through valid switches at appropriate voltages, the redundancy of the design is not exploited. The fault-caused imbalances can easily upset the driven load.
There is, thus, an unmet need in the art for a method and a device for driving a fault-tolerant three-level inverter.